The aim of the present study was to verify the validity of diagnostic criteria developed by the NDDG for the diagnosis of diabetes mellitus in old age. One hundred and fifty-one ambulatory old (range: 66-77 years) subjects (group A) underwent OGTT showing the following results: 33% normal, 12% non-diagnostic, 23% impaired glucose tolerance (IGT), 32% diabetic-type tolerance (DT). In addition, 84 subjects (group B) selected from 1978 to 1982 (42 aged 51-60 years, 30 aged 61-70, and 12 aged 71-80) with abnormalities of glucose tolerance during OGTT (IGT or DT) were asked to control their fasting plasma glucose every month during 1984. In group B a significant correlation between DT and subsequent development of fasting hyperglycemia was observed only in the subjects of the 6th and 7th decades of age. On the contrary, no subjects aged 71-80 years developed fasting hyperglycemia. The authors suggest that a high prevalence of abnormalities in glucose tolerance according to NDDG exists in old age which cannot be considered evidence of a true diabetes mellitus being unpredictive of a progression towards fasting hyperglycemia.